1. Field of the Invention
The present invention relates to a platinum temperature sensor and a method for producing the same, and in particular to a platinum temperature sensor in the case of which a platinum thin-film resistor, which is applied to a ceramic substrate, is used for temperature detection.
2. Description of Prior Art
A known platinum temperature sensor is shown in FIG. 3. In this known platinum temperature sensor, a platinum thin-film resistor 2 is applied to a ceramic substrate 4 which normally consists of aluminium oxide Al2O3. In the area in which the platinum thin-film resistor 2 is formed, a protective glaze 6 is provided on the surface of the ceramic substrate 4. The platinum layer, in which the platinum thin-film resistor 2 is normally formed in a meandering shape, is additionally patterned so as to include connecting areas 8 having lead wires 10 connected thereto in an electrically conductive manner for taking the sensor signal. For fixing the lead wires 10, a glaze 12 is provided.
The field of use of the platinum temperature sensor, which is shown in FIG. 3 and which is implemented in thin-film technology, is normally limited to 600xc2x0 C. In the last few years, there has, however, been an increasing demand for an embodiment that can be used for higher operating temperatures, which may exceed 1,000xc2x0 C. In the field of high-temperature sensors considerable efforts have therefore been made to provide platinum temperature sensors which are suitable to be used in such high temperature ranges. By purposefully selecting the composition of the protective glaze 6, it has already been possible to find satisfactory solutions for some cases of use, whereas in very particular fields of application, e.g. in special cases of use in the field of automotive engineering, the results do not satisfy all requirements. For example, the long-term stability of temperature sensors of the type described hereinbefore, especially when they have applied thereto a certain measurement current, which may e.g. be 5 mA, is not sufficiently guaranteed at the high temperatures occurring, viz. temperatures in the range of 800xc2x0 C. and 1,000xc2x0 C., since the protective glazes used may be decomposed electrochemically by the necessary measurement current at these high temperatures. The resultant material migration has a negative influence on the properties of the platinum so that the stability of the sensors and, consequently, the measuring accuracy are impaired.
By purposefully selecting the composition of the protective glazes, improvements could be achieved to a certain extent, but it was impossible to find protective glazes that withstand the electrochemical decomposition by the measurement current in the case of continuous loads in a temperature range of 1,000xc2x0 C. or more than 1,000xc2x0 C.
From the article xe2x80x9cFxc3xcgen von Technischen Keramiken mit Keramik-Grxc3xcnfolienxe2x80x9d by M. Neuhxc3xa4user et al., sfi/Ber. DKG 72 (1995) Nr. 1-2, methods for joining technical ceramics are known wherein ceramics green foils are used to connect two ceramic layers. A prerequisite for the joining method described there is that the sintering temperature of the ceramic green foil is below the sintering temperature of the ceramic to be jointed.
A temperature sensor having a platinum resistance layer, which is applied to a ceramic substrate and encapsulated by a glaze, is disclosed in DE 7629727 U1.
From De 37 33 192 C1 a PTC-temperature sensor is known herein a platinum resistor formed by means of a platinum thick-film technique is arranged between two ceramic green foils and an interlaminar binder layer, whereupon the two foils are laminated together by use of a pressure and a risen temperature and are sintered after that.
In DE 4445243 A1 a temperature sensor is described wherein three unprocessed ceramic substrates are laminated together, pressed and fired at 1.600xc2x0 C. in order to form a uniform piece. Before laminating a platinum resistor is arranged between two of the ceramic substrates.
It is the object of the present invention to provide a platinum temperature sensor which supplies reliable measurement results even in the case of continuous loads in a high temperature range, and a method for producing such a platinum temperature sensor.
According to a first aspect of the invention this object is achieved by a platinum temperature sensor comprising:
a ceramic substrate;
a platinum thin-film resistor applied to the ceramic substrate;
a ceramic cover layer; and
a connecting layer produced from a ceramic green sheet by pressure and temperature treatment, by means of which the ceramic cover layer is connected with the ceramic substrate in such a way that the platinum thin-film resistor is sealingly encapsulated with regard to the environment.
According to a second aspect of the invention this object is achieved by a platinum temperature sensor comprising:
a ceramic substrate;
a platinum thin-film resistor applied to the ceramic substrate;
a ceramic cover layer; and
a connecting layer made of a glaze that is applied to the ceramic substrate in a boarder area surrounding the platinum thin-film resistor by means of which the ceramic cover layer is connected with the ceramic substrate in such way that the platinum thin-film resistor is sealingly encapsulated with regard to the environment.
According to a third aspect of the invention, this object is achieved by a method for producing a platinum temperature sensor comprising the steps of:
providing a fired ceramic substrate with a platinum thin-film resistor applied to the main surface thereof;
applying a connecting layer made of a ceramic green layer to the main surface of the ceramic substrate; and
applying a fired ceramic cover layer to the connecting layer in such a way that the platinum thin-film resistor is sealingly encapsulated with regard to the environment by subjecting the ceramic green layer to a temperature treatment under application of pressure in such a way that the ceramic substrate and the ceramic cover layer will be connected.
The present invention is based on the knowledge that ceramic materials, especially aluminium oxide Al2O3 are insensitive to the above-described current induced decomposition, and thus this material that in addition is used in thin-film platinum temperature sensors as substrate material for the platinum film can advantageously also be used as protective material for capsulation of the platinum film layer. Thus, the problems of electrochemical decomposition and the connected deterioration of properties of the platinum temperature sensors are prevented, even when the protective cover is realized from a ceramic material.
On the one hand, the structured platinum film in the inventive platinum temperature sensor is therefore sufficiently protected against mechanical and chemical environmental influences. On the other hand the inventive platinum temperature sensor supplies reliable measurement results even in the case of continuous loads in a high temperature range of for example 1000xc2x0 C. or more than 1000xc2x0 C., since the above-described disadvantageous decomposition phenomenons do not occur in the case of the inventive platinum film temperature sensor.
In the inventive platinum temperature sensor the connecting layer is either applied to the whole area of the ceramic substrate provided with the platinum thin-film resistor or alternatively only on a border area of the same, so that the platinum thin-film resistor is surrounded by the connecting layer. If the connecting layer is only provided on the border area it is preferable to provide a sealing layer that can be made of glass, for example, on the side edges of the resulting layer structure. However, such a layer can also be provided when the connecting layer is applied to the whole area.